5. The Synapse

A microscopic gap between the axon terminal of one neuron and the next cell (dendrite, cell body, or muscle).

Electrical vs Chemical Synapses

Structure of a chemical synapse (synaptic knob)

Synaptic Transmission

• Impulse arrives → voltage-gated Ca²⁺ channels open → Ca²⁺ enters synaptic knob → vesicles fuse with the presynaptic membrane → neurotransmitter (e.g. acetylcholine) released by exocytosis into the synaptic cleft.
• Neurotransmitter diffuses across, binds receptors on the postsynaptic membrane, opening Na⁺ channels → depolarisation (EPSP) generates a new impulse.
• Neurotransmitter is removed by enzyme breakdown (e.g. acetylcholinesterase), reuptake, or diffusion.

Excitatory vs Inhibitory Synapses

• EPSP (excitatory): net influx of cations → depolarisation → may trigger an impulse; can involve spatial summation (different neurons, same time) or temporal summation (same neuron, rapid succession).
• IPSP (inhibitory): efflux of cations / influx of Cl⁻ → hyperpolarisation → prevents an impulse.

Functions of the Synapse

• Ensures one-way transmission; filters out weak/background stimuli; allows summation; protects effectors from overstimulation.